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Thread: casting small 20tooth gear from DIY wax copy.

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    Thumbs up casting small 20tooth gear from DIY wax copy.

    I'm attempting to create a few replicas of a small metric 20 tooth gear. The original gear is in the photos, and is black. The original part isn't off a shelf, and is hard to find in this exact size. It's not really a critical high speed, or load carrying gear so i figure ill make a good project out of it.

    Objective: Sand cast 2 aluminum gears from wax copies.

    It has taken me roughly 10-12 mold attempts to get the cast wax gears perfect. So far i have 3 perfect wax copies, and 2 others in "fair" condition. Every other gear had missing teeth, voids, air bubbles, or broke upon removal from mold. It has deff been a learning experience. Each time though i get better.

    The wax is a scentless colorless 100% parafin wax. It has a melting temp of 115-150 degrees, so i believe the molten aluminum should vaporize this stuff right away.

    I'm using Reprorubber the blue material you see in the pictures (Quicksetting Putty | Reprorubber®: Metrology-Grade Self-Curing Casting Solutions) to initially make the inverse image of the gear. It's a two part quick set putty that holds its exact shape extremely well after curing.

    I'm also using a spray on dry film parting agent, Polytetrafluoroethylene AKA Teflon, that works amazing! That was half the secret to get the mold to release from the Reprorubber without cracking the wax.

    I still have to clean up my final wax gears with an exacto knife.

    I plan on using some clean aluminum stock, and sand casting them using my lost foundry furnace, and sand. I plan on just burning the wax out. After casting, i'm going to clean them up by hand and then polish them using a vibratory metal tumbler.

    So far thats the plan. Any advice?

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    This isn't going to work too well. You can't just mix and match operations from two separate processes (lost wax and sand casting) and expect good results. You might be able to use your original gear as a sand-casting pattern, but it won't work well because of the sand texture in the teeth. It would still need machining to function well. Don't try pouring molten aluminum on a wax part buried in a mold - it can volatilize the wax rapidly and blow the metal back into your face. And if you try burning out your oil-bonded sand mold, it will just collapse into a heap. Plus, burning teflon is toxic.

    Even if this all did work okay (unlikely) the shrinkage of the wax plus the metal will make it too small. You could probably duplicate these gears just fine by pouring a 2-part plastic (Alumilite?) into your mold. And in the meantime, you can find someone who knows how to do casting (sand or lost wax) and have him/her show you how to do it safely and effectively.

    Andrew Werby
    ComputerSculpture.com — Home Page for Discount Hardware & Software



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    Andrew is correct in what hes telling you. The casting is going to shrink about .01 per inch. See if you can locate some metal casting plaster.



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    Gold Member diyengineer's Avatar
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    So if i sand cast the gear, its going to shrink .01". gotcha.

    If i use metal casting plaster, wont the aluminum also shrink .01" regardless?



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    I'm interested in this as well, and it's a great point about blowback with lost wax casting, but looking at the Tubalcain vids on youtube ("mrpete222" - the guys a legend!), he mentions a 2% shrinkage. Is this 2% shrinkage a fair constant regardless of the size of casting?

    Also, is there a blowback issue when using foam molds instead of wax?

    cheers,
    Ian

    Last edited by aarggh; 07-27-2012 at 03:27 AM.
    It's rumoured that everytime someone buys a TB6560 based board, an engineer cries!


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    Different alloys will shrink different amounts, but for aluminum in general it will be a little over 1%. However, if you're starting by molding a part and casting it in wax, then there's also the shrinkage of the rubber and the wax to consider. Different rubbers have different shrinkage factors; you should probably determine what yours is by experimentation. Waxes vary quite a bit as well; paraffin will shrink about 6% and machinable waxes shrink even more (and expand correspondingly when heated in a mold, which can cause cracking.) You also should plan on leaving a finishing margin, since most castings get their surfaces scraped away to some extent.

    There are investment plaster mixes made mostly for the dental industry that expand when heated, usually due to a proportion of cristobalite that's added. This will compensate for the shrinkage of the alloy used (mostly gold) but not for shrinkage of wax in a mold. These have to be used as directed, in the course of normal lost-wax investment casting.

    The "lost foam" casting process does rely on the hot metal to eliminate the foam in a cavity, but it only works because the amount of combustible material in expanded styrene foam is quite small compared to the amount of air it contains by volume. It also requires a more porous mold than normal sand-casting, to vent the fumes that result. Even so, it's a messy process, with lots of nasty smoke generated - don't try it indoors.

    Andrew Werby
    ComputerSculpture.com — Home Page for Discount Hardware & Software



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    Gold Member diyengineer's Avatar
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    Thumbs up

    Copy that. Im going to mill it out now.

    Solidworks 2010, CamBam, Cutviewer.

    Attached Thumbnails Attached Thumbnails -gear-cad-jpg   -gearcam-jpg  


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